The present invention relates generally to advanced technology thermal battery systems. More particularly, the present invention relates to sodium-sulfur thermal batteries for use in providing a high-density electrical energy source and most particularly it relates to improved methods for forming strong, leak-tight glass-graphite sealing systems used in such batteries.
The sodium-sulfur battery was first introduced in the mid 1960's. Since that time, there has been a great deal of interest in developing cell designs which are suitable for a wide variety of applications. Batteries which have been under development include those for use in automobiles and train locomotives. One such battery is described by J. L. Sudworth in the publication entitled "Sodium/Sulfur Batteries for Rail Traction", in the Record of the Tenth Intersociety Energy Conversion Engineering Conference, 1975, pages 616-620. Cell designs have also been investigated for producing batteries for storage of electricity for delayed use in order to level out the production rate of electricity and for space systems requiring high energy density. The sodium-sulfur battery is used as a secondary, that is, rechargeable battery. Its use as a primary (onetime discharge) battery would be unwarranted because of the cost, complexity and fragility involved in edge-sealing and incorporating a ceramic solid electrolyte into a battery design. In addition, there are other relatively inexpensive primary batteries of higher power density available in the marketplace.
Sodium-sulfur thermal batteries typically include a molten sodium electrode, a molten sulfur electrode and a relatively thin sheet of a solid ceramic sodium ion conducting electrolyte which serves as a separator between the sodium and sulfur electrodes. The electrolytic reaction occurs when sodium ions diffuse through the separator to react with the molten sulfur. Consequently, an important requirement for the separator is that is has a sufficiently high rate of diffusion for sodium ions to allow, during initial operation of the thermal battery, the formation of a sodium polysulfide electrolyte within the separator. To provide satisfactory mechanical strength for the thin ceramic sheet separators used in such a structure, it is typically bonded to an underlying porous graphite support plate.
The sodium-sulfur cell usually operates at a relatively high temperature (300.degree.-400.degree. C.) in order to maintain not only the sulfur and sodium, but also their reaction products, in a molten state. It is believed that the diffusion of the sodium ions into and through the separator occurs when the cell is heated to operating temperatures for the first time to produce a polysulfide gradient. This polysulfide gradient is composed of sodium sulfides having the formula Na.sub.2 S.sub.x wherein x is approximately five or less but greater than or equal to one. The composition of this gradient is believed to be: EQU Na.sub.2 S/Na.sub.2 S.sub.2 /Na.sub.2 S.sub.3 Na.sub.2 S.sub.4 Na.sub.2 S.sub.5
Na.sub.2 S is a solid at temperatures below 1000.degree. C. As a result, the solid Na.sub.2 S provides a solid barrier which prevents migration of liquid sulfur or sodium through the entire separator. At the same time, the remainder of the polysulfide gradient provides levels of ionic conductivity which are not possible with the previous solid ceramic materials. The use of a diffusion allowing separator in combination with a polysulfide gradient provides suitable liquid electrode separation, while also providing high rates of ionic conduction and resulting high electrical power output.
In use, it is found that a structure as described herein above reduces the electrical resistance of the sodium-sulfur cell, leading to low I.sup.2 R power loses and, therefore, high delivered power densities from the battery's cells. In this structure, the porous graphite backing, by supporting the beta" alumina separator, allows the use of a separator sheet that would otherwise be too thin to survive the thermal and mechanical stress generated during operation of the cell. At the same time, the porous graphite permits sodium to diffuse therethrough and contact the inner side surface of the separator so that the cell reaction can occur. The solid electrolyte is a critical part of the cell configuration because it must also provide separation of the liquid sodium from the liquid sulfur in order to prevent catastrophic cell failure. Solid electrolytes which have been used in sodium-sulfur batteries include beta"-(double prime) alumina and other sodium ion conducting ceramics or glasses, with beta"-alumina being the most popular solid electrolyte. Such batteries are effective for many applications, but when efforts are made to produce sodium-sulfur batteries having still higher power densities, it is found that the bond line between the beta" alumina separator and the porous graphite support tends to fail quickly in service because these materials suffer from having relatively low conductivity and, further, have coefficients of thermal expansion which are not well matched to other materials used in making the cell.
In an effort to overcome this problem, various materials to strengthen the bond line between the beta" alumina separator and graphite support and to seal the resultant battery structure have been investigated. Such materials have to be able to bond effectively to both the beta" alumina and graphite without significantly affecting the basic electrical reaction within the cell. One such material used for this purpose are borate glasses which bond to both the beta" alumina separator and to the graphite, without compromising the electrical integrity of the battery. In my copending U.S. Patent Application, Ser. No. 667,157, filed Mar. 11, 1991 and entitled "Glass Sealing Materials for Sodium-Sulfur Batteries and Batteries Made Therewith", the teachings of which are incorporated herein in their entirety, I have described one such borate sealing glass; said glass comprising a mixture of Na.sub.2 O, Cs.sub.2 O and B.sub.2 O.sub.3. However, in practice, it is found that in very high density applications, even this glass does not adhere strongly enough to the graphite support structure to make the glass to graphite bond sufficiently mechanically strong and leak-free for such service.